Here you can create a Bookmarklet in an easy way. Just type the code that you want executed and the link will be generated for you to drag to your Bookmarks bar.
You also have a jQuery script injection if you want to work with jQuery regardless of it being present on the target page. Just tick the checkbox to have it.Drag the Bookmarklet that will appear here to your bookmarks bar and then use it in any page you want
Interested in injecting some CSS instead? You should check CSS Inject!